A variety of different machines are used in the construction of roads and other infrastructure. Such machines (referred to collectively herein as “road-forming machines”) can include, without limitation, equipment that displaces, shapes, and/or forms the underlying roadbed, such as earthmovers, motor graders and the like. Road-forming machines can also include machines that deposit and/or form the actual roadway material, such as asphalt pavers, which deposit asphalt on a road surface, slipform pavers, which extrude or otherwise shape concrete on a road surface, and/or the like. Such road-forming machines are commonly controlled by advanced control systems that use high-precision positioning equipment to ensure that the machines shape the earth and/or the road surface consistent with an engineer's model for the project.
In some cases, such machines might include onboard positioning equipment, such as a global navigation satellite system (“GNSS”) receiver. Generally, however, such receivers do not provide the level of precision necessary for these types of projects. As a result, many such systems employ external positioning devices, such as total stations (of which many are commercially available from Trimble Navigation) to provide precise positioning information to the machine's control system. The use of such external devices, while providing precise positioning information, is not problem-free. For example, in many cases, a total station (or other positioning device) will need to be moved from one location to another, in order to continue to provide valid positioning information to the road-forming machine as the machine moves along the path of a road. Typically, the machine will have to cease operation while the positioning device is moved and calibrated at the new location. This interruption greatly reduces the efficiency of the construction operation, adding time and expense to the project. This inefficiency can be mitigated somewhat through the use of multiple positioning devices, but the inability of a control system to receive input from more than one device simultaneously means that the machine will still have to cease operation while it transitions from one positioning device to another.
Further, every time the positioning device is moved in such projects, or a control system switches from one positioning device to another, any error in the data received from either device can introduce a discontinuity in the road surface. Such errors can result from imprecision of the calibration of positioning devices, thermal effects (which can cause minor distortion of lasers used to determine the machine's position by the positioning devices), wind and other environmental effects, and the like. Continuity of the road surface is often of primary importance (to whatever degree of precision is appropriate, depending on the step in the process). Merely by way of example, in many cases, there is a relatively large degree of freedom in variance from the model of the project, so that a road surface a few centimeters higher lower than the elevation specified by the model is acceptable. On the other hand, a discontinuity of even a centimeter in the surface of a road can create significant issues (the least of which is a severe bump for any cars that travel the road). Hence, conventional systems further degrade efficiency by requiring additional processes to prevent or remedy such discontinuities.
Accordingly, there is a need for control systems for road-forming machines that provide more robust positioning control.